1. Field of the Invention
The present invention relates generally to the art of optical inspection of semiconductor wafers, and more specifically to a high throughput brightfield and darkfield wafer inspection system having image processing redirected from the mechanical and electronics segments of the inspection system to the optical domain.
2. Description of the Related Art
Semiconductor wafer inspection techniques have historically utilized brightfield illumination, darkfield illumination, or spatial filtering. Brightfield imaging is not generally sensitive to small particles. Brightfield imaging tends to scatter small particles away from the collecting aperture, thereby resulting in reduced returned energy. When a particle is small compared to the optical point spread function of the lens and small compared to the digitizing pixel, the brightfield energy from the immediate areas surrounding the particle typically contribute a large amount of energy. The small reduction in returned energy resulting from the small particle makes the particle difficult to detect. Further, the small reduction in energy from a small particles often masked out by reflectivity variations from the bright surrounding background such that small particles cannot be detected without numerous false detections. Additionally, if the small particle is on an area of low reflectivity, which may occur for some process layers on wafers and always for reticles, photomasks, and flat panel displays, the resultant background return is low and any further reduction due to the presence of a particle becomes very difficult to detect.
Newer systems utilize broadband brightfield imaging as opposed to traditional monochromatic or narrow band brightfield imaging. Broadband brightfield imaging minimizes contrast variations and coherent noise present in narrow band brightfield systems, but are not sensitive to small particles.
Darkfield imaging is employed to detect small particles on wafers, reticles, photomasks, flat panels, and other specimens. The advantage of darkfield imaging is that flat specular areas scatter very little light back toward the detector, resulting in a dark image. Darkfield illumination provides a larger pixel-to-defect ratio, permitting faster inspections for a given defect size and pixel rate. Darkfield imaging also permits fourier filtering to enhance signal to noise ratios.
Any surface features or objects protruding above the surface of the object scatter more light toward the detector in darkfield imaging. Darkfield imaging thus produces a dark image except where circuit features, particles, or other irregularities exist. Particles or irregularities are generally assumed to scatter more light than circuit features. However, while this assumption permits a thorough inspection for particles on blank and unpatterned specimens, in the presence of circuit features a darkfield particle inspection system can only detect large particles which protrude above the circuit features. The resulting detection sensitivity is not satisfactory for advanced VLSI circuit production.
While some attempts to improve darkfield performance have been attempted, such systems tend to have drawbacks, including drawbacks resulting from the very nature of darkfield illumination. For example, while brightfield illumination floods the entire field of view with light, darkfield illumination is confined to a narrow strip of light. Due to the nature of lasers, the application of light in darkfield illumination tends to be non-uniform and limits the amount of data which can be collected in a particular time period.
Some imaging systems currently available attempt to address problems associated with darkfield imaging. One instrument, manufactured by Hitachi, uses the polarization characteristics of the scattered light to distinguish between particles and normal circuit features, based on the assumption that particles depolarize light more than circuit features during the scattering process. When circuit features become relatively small (less than or on the order of the wavelength of light), the circuit can depolarize the scattered light as much as the particles. As a result, only larger particles can be detected without false detection of small circuit features.
Further, a system employing a combination of a monochromatic darkfield and a monochromatic brightfield imaging for wafer inspection is poorly adapted for inspecting Chemical Mechanaical Planarized (CMP) wafers, which often have film thickness variations and a grainy texture. Grainy texture, it should be noted, may also be a result of the metal grain structure of the wafer.
Another attempt to resolve problems associated with darkfield imaging positions the incoming darkfield illuminators such that the scattered light from circuit lines oriented at 0xc2x0, 45xc2x0, or 90xc2x0 are minimized. This method is generally effective on circuit lines, but light scattering from corners is still relatively strong. Further, detection sensitivity for areas having dense circuit patterns must be reduced to avoid the false detection of corners.
Prior systems for processing brightfield and darkfield data have relied on different processing techniques. The system of FIG. 1 illustrates a prior system which performed a full processing of a wafer using brightfield imaging followed by darkfield imaging and subsequent processing of the wafer. The problem with this mechanization is that throughput, or the time to process a single wafer, is generally poor, and it does not have the capability to use the combined results from both brightfield and darkfield imaging. As shown in FIG. 1, brightfield imaging 101 is performed on wafer 103, wherein the brightfield imaging has tended to be either monochromatic or narrow band imaging. The wafer image is received via sensor 104, which performs TDI, or Time Delay Integration, and a phase lock loop analog to digital conversion (PLLAD). Data is then directed to input buffer 105, which passes data to defect detector 107. Defect detector 107 uses delay 106 to perform a die-to-die or cell-to-cell comparison of the brightfield image processed wafer 103. The results are then passed to post processor 108 where brightfield defects are determined and passed. The wafer 103 is then illuminated using darkfield illumination 102 in the second run, and all subsequent processes are performed on the darkfield image. The result of this second run is a list of darkfield defects. The typical defect assessment performed by the defect detector and the post processor 108 is to set a threshold above which a feature is considered a defect, and only passing brightfield or darkfield results exceeding such thresholds. This does not completely account for the benefits associated with the combined effects of using brightfield and darkfield, and the amount of time necessary to perform all processing for a single wafer can be significant.
An alternative to the mechanization of FIG. 1 is presented in FIG. 2. The system of FIG. 2 illustrates simultaneous brightfield illumination 201 and darkfield illumination 202 of wafer 203. The simultaneous illumination is typically from a single illumination source, and the system receives the wafer images using dual TDI and PLLAD sensors 204 and 204xe2x80x2. Each sensor 204 and 204xe2x80x2 receives an image of wafer 203 and loads a signal representing that image into input buffer 205 or 205xe2x80x2, such as RAM. From buffer 205 or 205xe2x80x2 the system feeds data to defect detector 207 where data representing the wafer 203 is compared to similar or reference wafer characteristics under the control of delays 206 and 206xe2x80x2. Delays 206 and 206xe2x80x2 each provide timing for a die-to-die or cell-to-cell comparison by defect detector 207. Defect detector 207 uses information from both brightfield and darkfield illumination steps to determine the location of defects on wafer 203. The combined defect list from defect detector 207 is then evaluated using post processor 208 to identify pattern defects and particles.
The drawback in implementing the system illustrated in FIG. 2 is that individual TDI and PLLAD sensors 204 and 204xe2x80x2, input buffers 205 and 205xe2x80x2, and delays 206 and 206xe2x80x2 are highly sophisticated and expensive components, and the use of two of each such components significantly increases the cost of the entire machine. Further, performance of defect detector 207 and post processor 208 requires that all data be available and be evaluated at one time, which can cause significant delays and high processing costs. For example, it is not unusual to see brightfield imaging requiring a very short amount of time while darkfield imaging takes significantly longer. This system also uses monochromatic or narrowband brightfield imaging, which has a tendency to exhibit undesirable contrast variations and coherent noise problems as discussed above.
Spatial filtering is another technique used to enhance the detection of particles. With plane wave illumination, the intensity distribution at the back focal plane of a lens is proportional to the Fourier transform of the object. Further, for a repeating pattern, the Fourier transform consists of an array of light dots. Placement of a filter in the back focal plane of the lens to block out the repeating light dots permits filtering of the repeating circuit pattern and leaves only non-repeating signals, such as particles or other defects. The major limitation of spatial filtering is that only areas having repeating areas or blank areas may be inspected.
There has been little interest in combining brightfield and darkfield techniques due to a lack of understanding of the advantages presented by such a technique. All of the machines currently available employing monochromatic brightfield/darkfield imaging use a single light source for both brightfield and darkfield illumination and do not use a combination of brightfield and darkfield images to determine defects.
Microscopes exist on the market today which combine both monochromatic brightfield and darkfield illumination, and such microscopes have a single light source and provide both illuminations simultaneously, thus making it impossible to separate the brightfield and darkfield results. Such mechanizations simply result in a combined full-sky illumination.
A further limitation of prior systems is that the illumination sources tend to be fixed in place, which also fixes the ability of the system to pick up defects in surfaces or specimens having different physical properties. Typically, a video camera is positioned above the specimen and light is applied to the specimen at a predetermined angle. The application of light to a particle tends to scatter the light, which is then detected by the video camera. If the specimen contains an irregular surface configuration, such as excess material or a semiconductor pattern, the fixed angle of the light source may not optimally scatter the applied light, inhibiting the ability to detect defects. Even for a wafer having a regular semiconductor pattern, orientation of the illumination source provides a different return when a pattern feature is oriented at 0xc2x0, 45xc2x0, or 90xc2x0. Also, the support mechanisms and circuitry associated with the light source tend to be large and bulky, thereby impeding the repositioning capability of the light source.
Another problem with brightfield/darkfield imaging is the use of imaging devices within the same physical space. Components associated with brightfield imaging are generally used for darkfield imaging as well, and several overlapping components exist when using both forms of illumination and detection. However, due to the optical, physical, and other characteristics of components used in brightfield/darkfield imaging, some components tend to provide advantages with one form of illumination and disadvantages for the other illumination scheme. The minimization of the disadvantages associated with a form of imaging improves the ability to detect problems associated with individual specimens.
Another problem associated with wafer inspection systems is the control of light level. Control of light level is particularly complex and critical where a high level of light collection efficiency is desired, and where the gain of the detector is not readily controlled. Prior systems for providing light level control for wafer inspection include providing absorbing glass in the illumination path, and control over the output energy of the laser. These systems either do not perform sufficiently and/or are too costly or complex to use efficiently.
It is therefore an object of the current invention to provide a system for detecting defects on a wafer, the system having the ability to detect defects beyond those detectable using monochromatic or narrowband brightfield imaging alone. Inherent in such a system would be the ability to minimize contrast variations and coherent noise problems.
It is another object of the current invention to provide a system for detecting defects which has the ability to detect small particles, including particles having a size smaller than the wavelength of light, with a minimum number of false detections. The system should provide for a minimum number of components to decrease overall cost and provide for maximum throughput of specimens.
It is yet another object of the current invention to provide a system which provides the ability to perform an accurate inspection of Chemical Mechanaical Planarized (CMP) wafers, or other specimens having film thickness variations or grainy textures.
It is still another object of the current invention to provide a system for detecting defects on a wafer wherein the system has the ability to optimize the incidence of light reflected from specimens having various surface characteristics.
It is still another object of the current system to provide both brightfield and darkfield illumination using a minimum number of components in a minimum amount of physical space while simultaneously minimizing adverse effects associated with brightfield and darkfield illumination.
It is still another object of the current invention to provide an efficient method or apparatus for light level control in the wafer inspection system.
According to the present invention, there is provided a broadband brightfield/darkfield wafer inspection system. Broadband brightfield illumination illuminates a wafer, and data from this illumination is captured by a sensor. The sensor is preferably a TDI sensor having PLLAD capability, but other sensors, such as a non-integrating CCD or linear sensor may be employed. The sensor thereupon loads a signal representative of the image into an input buffer, which feeds data to a defect detector, where the broadband brightfield data from the sample being inspected is compared to a similar sample or reference wafer using timing control from a delay. The defect detector signals initiation of darkfield illumination of the wafer.
The sensor captures illumination resulting from darkfield illumination and loads a signal representative of the image into the input buffer, which feeds data to the defect detector. Darkfield data in a similar manner to broadband brightfield data using the delay. Darkfield illumination data from the defect detector is then passed to post processor.
The defect detector signals commencement of the darkfield imaging based on the type of wafers presented and the expected timing associated with the wafers. Broadband brightfield and darkfield data does not overlap along the system path, and the time associated with processing the combined broadband brightfield and darkfield data is minimized.
The defect detector includes a 2D histogram circuit which forms a two dimensional histogram of the defect data with brightfield differences plotted on one axis and darkfield differences plotted on the orthogonal axis. The histogram information is then applied to a dual mode defect decision algorithm, which sizes and locates defects resulting from the brightfield and darkfield inspections. The post processor evaluates the quality and importance of the detected defects. Ideally, the dual mode defect decision algorithm and post processor exclude predictable variations without identifying them as defects, and identifies other responses outside an expected range to be defects, and broadband brightfield and darkfield data may be combined and used to accomplish this intent.
Darkfield radiation is provided by two adjustable height laser beams. The laser beams illuminate the surface of the wafer at an angle of approximately 6 to approximately 39 degrees. The first laser is oriented at an azimuth angle 45 degrees greater than the orientation of the manhattan geometry on the wafer, and the second laser is oriented at an azimuth angle 45 degrees less than the manhattan geometry on the wafer, or 90 degrees offset from the first laser.
Darkfield illumination within the system accommodates three elevation angles to provide varying ability to illuminate the wafer. At the high grazing angle setting, 39 degrees, the best sensitivity for low noise wafers such as smooth film and early etch specimens is available. The low grazing angle setting, 6 degrees, provides some attenuation of noise from the wafer pattern or from wafer roughness. The 20 degree grazing angle illumination is a compromise setting which offers a tradeoff between the sensitivity benefit of the 39 degree angle and the noise reduction of the 6 degree angle.
While the elevation grazing angle settings include 6, 20, and 39 degree settings, the mechanization of the current invention provides for a continuously variable angular offset, and thus the elevation grazing angle may vary anywhere from approximately five to approximately 45 degrees.
The apparatus providing the adjustable angle uses a rotating cylindrical lens to control the angular orientation of the laser spot, a pixel size changer, and an adjustable mirror. The angle of the adjustable mirror is altered to change the angle of incidence of each of the lasers on the wafer. The position of the cylindrical lens is modified to compensate for that change and maintain the elliptical spot in the correct position relative to the surface of the wafer and sensor.
The system can compensate for mirror rotation by moving, rotating, or moving and rotating the cylindrical lens. The preferred method is to translate the adjustable mirror in the vertical direction, normal to the wafer, to position the illumination spot into the sensor field of view, to rotate the cylindrical lens to properly orient the ellipse, and finally to move the cylindrical lens to obtain desired ellipticity.
The brightfield beamsplitter provided is removable, and preferably replaced with a blank, or glass, when performing darkfield illumination. This allows more light to pass to sensor and permits greater levels of detection in darkfield imaging. An alternative method for producing the same result is to perform brightfield imaging in a selected color light spectrum and performing darkfield in a different frequency light spectrum, such as red being selected for brightfield illumination and green for darkfield illumination.
Light level control for the system is provided by a dual polarizer first stage, wherein the polarizers are rotated relative to one another to control the intensity of the beam passing through them. The relative rotation of the polarizers provides variation of the beam intensity in a continuous manner, preferably without varying the polarization of the beam. Rotation of the second polarizer controls the balance between the two output channels. Light exiting from the second polarizer passes through a filter, which is preferably a discrete glass filter, and which absorbs a portion of the light and comprises the second stage of light control.
The beam then passes through a polarizing beamsplitter, which divides the light into first and second channels. The second channel is further reflected and polarized, as needed, and both beams thereafter illuminate the substrate. Both beams preferably have equal intensity as they impinge on the substrate surface.
Other objects, features, and advantages of the present invention will become more apparent from a consideration of the following detailed description and from the accompanying drawings.